Copywriting is the most ubiquitously undervalued service in Internet Marketing, in terms increasing the bottom-line. Too many product vendors think they can do it themselves and subsequently wonder why their copy is not converting.
A professional sales copy can make the difference between selling tens or hundreds of a particular offer, and selling thousands if not millions.
I have written sales copy for some of the top 'gurus' in the IM industry, and just swapping out their old copy for mine--engendered a 300% increase in conversion.
Seriously, don't underestimate the value of a top notch copywriter.